It depends on where you copied it
from. And if there were other copies in other classpaths.
JDBC drivers should almost always be copied to the TOMCAT_HOME/lib directory and not made part of the WARs, however. Take that as a near-universal rule. For one thing, the connection pooling is normally provided by the server, so the driver has to be in the server classpath, which means TOMCAT_HOME/lib.
It might be just a co-incidence that you got the "Connection reset". Network-related problems usually cause that. However, if you DID have 2 copies of the driver in the system, differing access paths could have resulted in internal inconsistencies, like where one copy sent out the request, and the other copy listened for it (but didn't have the context of the sender!).
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.